The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The usual procedure involves (see U.S. Pat. No. 2,297,691) placing a uniform electrostatic light image on the charged photoconductive surface to discharge the irradiated areas, and this leaves the remainder of the surface in a charged condition. The resulting electrostatic image is rendered visible by depositing on it a finely divided electroscopic marking material which is held electrostatically to the charged areas of the surface. This powder image may then be transferred to a sheet of paper and permanently affixed to it by heat fusion or in some other fashion.
Development of an electrostatic latent image may also be achieved by liquid rather than dry developer materials. In conventional liquid development, commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid imaging material dispersed therein together with dissolved resinous binding material contacts the photoconductive surface in both charged and uncharged areas. The dispersed imaging particles migrate toward the charged areas of the surface, and this migration of charged particles results in their deposition on the charged areas of the photoconductor, thus rendering the electrostatic image visible. The separation of the suspended particles (e.g. carbon black) from the insulating liquid and the binder dissolved therein is incomplete. It is estimated, in the case of carbon black, that the deposited carbon particles contain roughly ten times their weight of insulating liquid absorbed thereon. It is this absorbed liquid that is responsible for the fixing of the image upon evaporation of the volatile portion of the solution.
A typical conventional liquid developer (see for example U.S. Pat. No. 3,890,240) will contain 0.044% of carbon black and 0.36% of resinous binder (a pigment to binder ratio of 1 to 8). However, the actual amount of binder carried by one gram of carbon black and deposited on the charged areas by electrophoresis will be only 0.36 g. This pigment to binder ratio is inadequate for proper fixing of the image, particularly when the image is to be transferred from the photoconductive surface to a sheet of plain paper. Moreover, the amount of resinous binder in the liquid developer cannot be increased substantially since electrophoretic liquid developers require low viscosity to achieve rapid development without considerable background deposition. This invention is concerned with increasing the amount of resinous binder deposited with the imaging pigment in a low viscosity liquid developer adapted for rapid development.